1. Field of the Invention
The present invention generally relates to tape reels for magnetic tape cartridges and more particularly, to a tape reel suitable for use in a single-reel type magnetic tape cartridge in which a single tape reel is received in a cartridge casing.
2. Description of the Prior Art
A tape reel is constituted by a reel body in which a bottomed tubular hub opening at its upper end and a lower flange are formed integrally, and an upper flange mounted on the upper end of the hub. On upper and lower surfaces of an outer peripheral edge portion of a bottom wall of the hub, a plurality of locking teeth engageable with a reel retainer and a plurality of driving teeth engageable with a driving shaft of a tape drive are, respectively, formed circumferentially so as to project therefrom. A tape reel of this type is known from, for example, Japanese Patent Laid-Open Publication No. 11-238352 (1999) referred to as “a prior art document 1”.
Such reel body is usually obtained by injection molding molten resin from a gate position provided at a center on the upper surface of the bottom wall of the hub. Regarding molding of the reel body, several proposals for devising the gate position, fluidity of the molten resin, etc. have been advanced. For example, a technique in which the gate position is provided at an inner peripheral edge portion on a lower surface of the lower flange for the purpose of shortening a molding cycle and facilitating mold release is proposed in Japanese Patent Laid-Open Publication No. 7-161167 (1995) referred to as “a prior art document 2”, while another technique in which fluidity of the resin in a cavity is improved so as to eliminate defective molding is proposed in Japanese Patent Laid-Open Publication No. 2002-367331 referred to as “a prior art document 3”. Meanwhile, in the above prior art document 2, a reel body is formed by a lower flange and a bottomed tubular hub opening downwardly. On the other hand, in the above prior art document 3, a locking member for locking a reel body is subjected to molding.
Several kinds of single-reel type magnetic tape cartridges are used as data backup mediums for office computers. In response to the recent trend to larger capacity of hard disks subjected to backup, there is a demand for not only higher recording density of the magnetic tape cartridge of this kind but higher read rate and write rate of recording signals. In order to raise recording density of the recording signals, wavelength of the recording signals is further shortened and the track pitch is set to be narrower. In order to ensure and speed up signal recording in a tape system in which recording density of the recording signals is raised in this way, Japanese Patent Laid-Open Publication No. 2002-269711 referred to as “a prior art document 4” proposes a servo track system in which servo signals are preliminarily recorded on a magnetic tape such that positional control of a magnetic head array is performed precisely on the basis of the servo signals. In this known servo track system in which read and write are performed at higher density and at higher rate than other conventional tape systems, it is essential due to the narrow track pitch to accurately position the magnetic tape relative to the magnetic head. Thus, reliability of read and write of the signals is greatly affected by scatter of shape and dimensions of the tape reel and stable running of the magnetic tape naturally. For example, a rotational plane of the tape reel tilts periodically at the time of read and write of the signals due to defective shape of the tape reel. Alternatively, if there is nonuniformity in rotational balance, error of read and write may happen. In other words, in this known servo track system, molding accuracy of the tape reel, which has offered substantially no problem in other conventional tape systems may cause such defects as off-track or the like and thus, will constitute one of deciding factors in attaining far higher read rate and write rate in the future.
As described above, in a reel body, the bottomed tubular hub opening at its upper end and, the lower flange are formed integrally. When thicknesses of respective portions of the reel body are compared with each other, the bottom wall of the hub fundamentally has a thickness of 1.2 mm, while an overall thickness of the bottom wall of the hub at its portion formed with the locking teeth reaches 4.5 mm which is a little less than four times the thickness of the bottom wall of the hub. Meanwhile, it is theoretically considered that a speed (time) at which the molten resin sets is proportional to a square of the thickness. According to this consideration, the period required for setting of the portion of the bottom wall of the hub, which is formed with the locking teeth, is about 14 times the period required for setting of the bottom wall of the hub. This difference in setting time may form a cause leading to production of sink marks, i.e., shrinkage recesses due to setting, thereby resulting in scatter of flatness or roundness.
In order to avoid such molding defects as sink marks when an item having thick portions scattered to several locations as described above is molded, a molding technique is known in which even after injected molten resin has been filled into a cavity, a predetermined injection pressure is maintained continuously such that the molten resin is supplemented in response to shrinkage of the volume upon its setting. However, in the method in which the reel body is molded by providing the gate position at the center on the upper surface of the bottom wall of the hub, even if molding is performed while the resin is being supplemented, it is impossible to effectively eliminate sink marks occurring at the locations for forming the locking teeth and the driving teeth as well as other molding defects, so that it is difficult to obtain the tape reel having high molding accuracy.
The locking teeth and the driving teeth, which are formed more thickly than other portions of the tape reel as described above, are continuous with the gate position via the bottom wall of the hub, which is as thin as 1.2 mm. Hence, even if the predetermined injection pressure is maintained after filling of the molten resin, a portion of the bottom wall of the hub, which is disposed inside the locking teeth, sets prior to setting of the locking teeth and the driving teeth and thus, supplement of the resin upon shrinkage of the volume cannot be performed sufficiently at the thick portions. If the locking teeth or the driving teeth cannot be molded at high shape accuracy, it is impossible to avoid deterioration of cylindricalness of the hub and planar distortion degree of the lower flange and thus, it is difficult to obtain the tape reel suitable for the servo track system. Especially, in case molding accuracy of the driving teeth engageable with the driving shaft of the tape drive is low, the magnetic tape cannot be supplied stably, thereby resulting in an obstacle to higher read rate and write rate.